The idea that planets beyond Earth might
host life is an ancient one, though historically it was framed by philosophy
as much as physical
science1 .
The late 20th century saw two breakthroughs in the field. To begin with, the
observation and robotic exploration
of other planets and moons within the solar system has provided critical
information on defining habitability criteria and allowed for substantial
geophysical comparisons between the Earth and other bodies. The discovery of extrasolar
planets—beginning in 1995 and accelerating thereafter—was the second
milestone. It confirmed that the Sun is not unique in hosting planets and
expanded the habitability research horizon beyond our own solar system.

Suitable
star systems

An understanding of planetary habitability
begins with stars.
While bodies that are generally Earth-like may be plentiful, it is just as
important that their larger system be agreeable to life. Under the auspices of
SETI's Project
Phoenix, scientists Margaret
Turnbull and Jill
Tarter developed the "HabCat"
(or Catalogue of Habitable Stellar Systems) in 2002. The catalogue was formed
by winnowing the nearly 120,000 stars of the larger Hipparcos
Catalogue into a core group of 17,000 "HabStars," and the
selection criteria that were used provide a good starting point for
understanding which astrophysical factors are necessary to habitable planets.

Spectral
class

The spectral class of a star indicates its
photospheric
temperature, which (for main-sequence
stars) correlates to overall mass. The appropriate spectral range for
"HabStars" is presently considered to be "early F" or
"G", to "mid-K". This corresponds to temperatures of a
little more than 7,000K
down to a little more than 4,000K; the Sun (not coincidentally) is directly in
the middle of these bounds, classified as a G2 star. "Middle-class"
stars of this sort have a number of characteristics considered important to
planetary habitability:

They live at least a few billion years, allowing life a chance to
evolve. More luminous
main-sequence stars of the "O," "B," and "A"
classes usually live less than a billion years and in exceptional cases
less than 10 million [2]2 .

Liquid water may exist on the surface of planets orbiting them at a
distance that does not induce tidal
lock (see next section and 3.1).

These stars are neither "too
hot" nor "too cold" and live long enough that life has a chance
to begin. This spectral range likely accounts for between 5 and 10 percent of
stars in the local Milky Way galaxy. Whether fainter late K and M class
("red
dwarf") stars are also suitable hosts for habitable planets is
perhaps the most important open question in the entire field of planetary
habitability given that the majority of stars fall within this range; this is
discussed extensively below.

A range of theoretical habitable
zones with stars of different mass (our solar system in middle) - a stable
habitable zone

The habitable zone (HZ) is a theoretical
shell surrounding a star in which any planets present would have liquid water
on their surfaces. After an energy source, liquid water is considered the most
important ingredient for life, considering how integral it is to all
life-systems on Earth. This may reflect the bias of a water-dependent species,
and if life is discovered in the absence of water (for example, in a liquid-ammonia
solution), the notion of an HZ may have to be greatly expanded or else
discarded altogether as too restricting.

A "stable" HZ denotes two
factors. First, the range of an HZ should not vary greatly over time. All
stars increase in luminosity as they age and a given HZ naturally migrates
outwards, but if this happens too quickly (for example, with a super-massive
star), planets may only have a brief window inside the HZ and a
correspondingly weaker chance to develop life. Calculating an HZ range and its
long-term movement is never straightforward, given that negative feedback
loops such as the carbon
cycle will tend to offset the increases in luminosity. Assumptions made
about atmospheric conditions and geology thus have as great an impact on a
putative HZ range as does Solar evolution; the proposed parameters of the
Sun's HZ, for example, have fluctuated greatly.

Secondly, no large-mass body such as a gas
giant should be present in or relatively close to the HZ, thus disrupting the
formation of Earth-like bodies. If, for example, Jupiter
had appeared in the region that is now between the orbits of Venus
and Earth, the two smaller planets would almost certainly not have formed. It
was once assumed that the inner-rock planets, outer-gas giants pattern
observable in the solar system was likely to be the norm elsewhere, but
discoveries of extrasolar
planets have overturned this notion. Numerous Jupiter-sized bodies have
been found in close orbit about their primary, disrupting potential HZs.
Present data for extrasolar planets is likely skewed towards large planets in
close eccentric orbits because they are far easier to identify; it remains to
be seen which type of solar system is the norm.

Low
stellar variation

Changes in luminosity
are common to all stars, but the severity of such fluctuations covers a broad
range. Most stars are relatively stable, but a significant minority of
variable stars often experience sudden and intense increases in luminosity and
consequently the amount of energy radiated toward bodies in orbit. These are
considered poor candidates for hosting life-bearing planets as their
unpredictability and energy output changes would negatively impact organisms.
Most obviously, living things adapted to a particular temperature
range would likely be unable to survive too great a temperature deviation.
Further, upswings in luminosity are generally accompanied by massive doses of gamma
ray and X-ray
radiation which might prove lethal. Atmospheres
do mitigate such effects (an absolute increase of 100 percent in the Sun's
luminosity would not necessarily mean a 100 percent absolute temperature
increase on Earth), but atmosphere retention might not occur on planets
orbiting variables, because the high-frequency energy buffetting these bodies
would continually strip them of their protective covering.

The Sun, as in much else, is benign in
terms of this danger: the variation between solar max and minimum is roughly
0.1 percent over its 11-year solar
cycle. There is strong (though not undisputed) evidence
that even minor changes in the Sun's luminosity have had significant effects
on the Earth's climate well within the historical era; the Little
Ice Age of the mid-second millennium, for instance, may have been caused
by a relatively long-term decline in the sun's luminosity [5] .
Thus, a star does not have to be a true variable for differences in luminosity
to affect habitability. Of known "solar
twins," the one that most closely resembles the Sun is considered to
be 18
Scorpii; interestingly (and unfortunately for the prospects of life
existing in its proximity), the only significant difference between the two
bodies is the amplitude of the solar cycle, which appears to be much greater
on 18 Scorpii.

High
metallicity

While the bulk of material in any star is hydrogen
and helium,
there is a great variation in the amount of heavier elements (metals)
stars contain. A high proportion of metals in a star correlates to the amount
of heavy material initially available in protoplanetary
disks. A low amount of metal significantly decreases the probability that
planets will have formed around that star, under the solar
nebula theory of planetary
systems formation. Any planets that did form around a metal-poor star
would likely be low in mass, and thus unfavorable for life. Spectroscopic
studies of systems where exoplanets
have been found to date confirm the relationship between high metal content
and planet formation: "stars with planets, or at least with planets
similar to the ones we are finding today, are clearly more metal rich than
stars without planetary companions [7] ."
High metallicity also places a requirement for youth on hab-stars: stars
formed early in the universe's
history have low metal content and a correspondingly lesser likelihood of
having planetary companion.

Binary
systems

Current estimates suggest that at least
half of all stars are in a binary
system[8] ,
which further complicates a delineation of habitability. The separation
between stars in a binary may range from less than one astronomical
unit (AU, the Earth-Sun distance) to several hundred. In latter instances,
the gravitational effects will be negligible on a planet orbiting an otherwise
suitable star and habitability potential will not be disrupted unless the
orbit is highly eccentric (see Nemesis,
for example). However, where the separation is significantly less, a stable
orbit may be impossible. If a planet’s distance to its primary exceeds about
one fifth of the closest approach of the other star, orbital stability is not
guaranteed [9] .
Whether planets might form in binaries at all had long been unclear, given
that gravitational forces might interfere with planet formation. Theoretical
work by Alan
Boss at the Carnegie
Institute has shown that gas giants can form around stars in binary
systems much as they do around solitary stars [10] .

Alpha
Centauri, the nearest star system to the Sun, underscores the fact that
binaries need not be discounted in the search for habitable planets. Centauri
A and B have an 11 AU distance at closest approach (23 AU mean), and
both should have stable habitable zones. A study of long-term orbital
stability for simulated planets within the system shows that planets within
approximately three AU of either star may remain stable (i.e. the semi-major
axis deviating by less than 5 percent). The HZ for Centauri A is
conservatively estimated at 1.2 to 1.3 AU and Centauri B at 0.73 to 0.74
— well within the stable region in both cases.

Planetary
characteristics

The chief assumption about habitable
planets is that they are terrestrial.
Such planets, roughly within one order of magnitude of Earth mass, are
primarily composed of silicate
rocks and have not accreted the gaseous outer layers of hydrogen
and helium
found on gas
giants. That life could evolve in the cloud tops of giant planets has not
been decisively ruled out 4 ,
though it is considered unlikely given that they have no surface and in most,
their gravity is enormous [12] .
The natural satellites of giant planets, meanwhile, remain perfectly valid
candidates for hosting life [13] .

In analyzing which environments are likely
to support life a distinction is usually made between simple, unicellular
organisms such as bacteria
and archaea
and complex metazoans (animals). Unicellularity necessarily precedes
multicellularity in any hypothetical tree of life and where single-celled
organisms do emerge there is no assurance that this will lead to greater
complexity 6 .
The planetary characteristics listed below are considered crucial for life
generally, but in every case habitability impediments should be considered
greater for multicellular organisms such as plants and animals versus
unicellular life.

Mars,
with its thin atmosphere, is colder than Earth would be at a similar distance
from the Sun

Mass

Low-mass planets are poor candidates for
life for two reasons. First, their lesser gravity
makes atmosphere
retention difficult. Constituent molecules
are more likely to reach escape
velocity and be lost to space when buffeted by solar
wind or stirred by collision. Planets without a thick atmosphere lack the
matter necessary for primal biochemistry,
have little insulation and poor heat
transfer across their surfaces (for example, Mars
with its thin atmosphere is colder than the Earth would be at similar
distance) and lesser protection against high-frequency radiation
and meteoroids.
Secondly, smaller planets have smaller diameters
and thus higher surface-to-volume ratios than their larger cousins. Such
bodies tend to lose the energy left over from their formation quickly and end
up geologically
dead, lacking the volcanoes,
earthquakes
and tectonic
activity which supply the surface with life-sustaining material and the
atmosphere with temperature moderators like carbon
dioxide.

"Low mass" is partly a relative
label; the Earth is considered low mass when compared to the Solar System's gas
giants, but it is the largest, by diameter and mass, and densest of all
terrestrial bodies 5 .
It is large enough (along with Venus) to retain an atmosphere through gravity
alone and large enough that its molten core remains a heat engine, driving the
diverse geology of the surface. Mars, by contrast, is nearly (or perhaps
totally) geologically dead and has lost much of its atmosphere [14] .
Thus, it would be fair to infer that the lower mass limit for habitability
lies somewhere between Mars and Earth-Venus. Exceptional circumstances do
offer exceptional cases: Jupiter's
moon Io
(smaller than the terrestrial planets) is volcanically dynamic because of the
gravitational stresses induced by its orbit; neighbouring Europa
may have a liquid ocean underneath a frozen shell due also to energy created
in its orbiting a gas giant; Saturn's
Titan,
meanwhile, has an outside chance of harbouring life as it has retained a thick
atmosphere and bio-chemical reactions are possible in liquid methane on its
surface. These satellites are exceptions, but they prove that mass as a
habitability criterion cannot be considered definitive.

Finally, a larger planet is likely to have
a large iron core. This allows for a magnetic
field to protect
the planet from the solar
wind, which otherwise tends to strip away the planetary atmosphere and to
bombard living things with ionised particles. Mass is not the only criterion
for producing a magnetic field — as the planet must also rotate fast enough
to produce a dynamo
effect within its core [15]
— but is a significant component of the process.

Orbit and
rotation

As with other criteria, stability is the
critical consideration in determining the effect of orbital and rotational
characteristics on planetary habitability. Orbital
eccentricity is the difference between a planet's closest and farthest
approach to its primary. The greater the eccentricity the greater the
temperature fluctuation on a planet's surface. Although adaptive, living
organisms can only stand so much variation, particularly if the fluctuations
overlap both the freezing
point and boiling
point of the planet's main biotic solvent (i.e., water). If, for example,
Earth's oceans were alternately boiling off into space and freezing solid, it
is difficult to imagine life as we know it having evolved. Fortunately, the
Earth's orbit is almost wholly circular, with an eccentricity of less than
0.02; other planets in our solar system (with the exception of Pluto
and to a lesser extent Mercury)
have eccentricities that are similarly benign. Data collected on the orbital
eccentricities of extrasolar planets has surprised most researchers and
reduced the expected extraterrestrial possibilities for life: 90% have an
orbital eccentricity greater than that found within the solar system, and the
average is fully 0.25, although this could very easily be the result of sample
bias due to increased star 'wobble' caused by the planet's eccentricity.

A planet's movement around its rotational
axis must also meet certain criteria if life is to have the opportunity to
evolve.

The day-night cycle must not be overlong. If a day takes years, the
temperature differential between the day and night side will be
pronounced, and problems similar to those noted with extreme orbital
eccentricity will come to the fore.

The planet must have moderate seasons. If there is little axial
tilt (relative to the perpendicular to the ecliptic),
seasons will not occur and a main stimulant to biospheric dynamism will
disappear; such planets will generally be colder than they would be with a
tilt. If a planet is radically tilted seasons will be extreme and make it
more difficult for a biosphere to achieve homeostasis.
The exact effects of these changes can only be computer modelled at
present, and studies have shown that even extreme tilts of up to 85
degrees do not absolutely preclude life "provided [it] does not
occupy continental surfaces plagued seasonally by the highest temperature [17] ."

The rotational "wobble" must not be pronounced. Precession
on Earth occurs over a 23 000 year cycle; if this period were radically
shorter or if the wobble were more extreme, drastic climatic changes would
again affect habitability.

The Earth's moon appears to play a crucial
role in moderating the Earth's climate by stabilising the axial tilt. It
has been suggested that a chaotic tilt may be a "deal-breaker" in
terms of habitability— i.e. a satellite the size of the moon is not only
helpful but required to produce stability [18] .
This position remains controversial 7 .

Geochemistry

It is generally assumed that any
extraterrestrial life that might exist will be based on the same fundamental
chemistry as found on Earth,
as the four elements most vital for life, carbon,
hydrogen,
oxygen, and nitrogen,
are also the most common chemically reactive elements in the universe. Indeed,
simple biogenic compounds, such as amino acids, have been found in meteorites
and in interstellar
space. These four elements by mass make up over 96 percent of Earth's
collective biomass.
Carbon has an unparalleled ability to bond with itself and to form a massive
array of intricate and varied structures, making it an ideal material for the
complex mechanisms that form living cells. Hydrogen and oxygen, in the form of
water, compose the solvent in which biological processes take place and in
which the first reactions occurred that led to life's emergence. The energy
contained in the powerful covalent bond between carbon and hydrogen, released
from the breakdown of carbohydrates,
is the fuel of all complex lifeforms. These four elements together make up amino
acids, which in turn are the building blocks of proteins,
the substance of living tissue.

Relative abundance in space does not
always mirror differentiated abundance within planets; of the four life
elements, for instance, only oxygen is present in any abundance in the Earth's
crust [19] .
This can be partly explained by the fact that many of these elements, such as hydrogen
and nitrogen,
along with their most basic compounds, such as carbon
dioxide, carbon
monoxide, methane,
ammonia,
and water, are
gaseous at warm temperatures. In the hot region close to the Sun,
these volatile compounds could not have played a significant role in the planets'
geological formation. Instead, they were trapped as gases underneath the newly
formed crusts, which were largely made of rocky, involatile compounds such as silica
(a compound of silicon
and oxygen, accounting for oxygen's relative abundance). Outgassing
of volatile compounds through the first volcanoes would have contributed to
the formation of the planets' atmospheres.
The Miller
experiments showed that, with the application of energy, amino
acids can form from the synthesis of the simple compounds within a
primordial atmosphere [20] .

Even so, volcanic outgassing could not
have accounted for the amount of water in Earth's oceans [21] .
The vast majority of the water, and arguably of the carbon, necessary for life
must have come from the outer solar system, away from the Sun's
heat, where it could remain solid. Comets
impacting with the Earth
in the Solar system's early years would have deposited vast amounts of water,
along with the other volatile compounds life requires (including amino acids)
onto the early Earth, providing a kick-start to the evolution of life.

Thus, while there is reason to suspect
that the four "life elements" ought be readily available elsewhere,
a habitable system likely also requires a supply of long-term orbiting bodies
to seed inner planets. Without comets there is a possibility that life as we
know it would not exist on Earth. The possibility also remains that other
elements beyond those necessary on Earth will provide a biochemical basis for
life elsewhere; see alternative
biochemistry.

Other
considerations

Gaia

The Gaia
hypothesis, a class of scientific models of the geo-biosphere pioneered by
Sir James
Lovelock in 1975,
argues that life as a whole fosters and maintains suitable conditions for
itself by helping to create a planetary environment suitable for its
continuity. In other words, an effect of "survival
of the fittest" is that the most successful life forms change the
composition of the air, water, and soil in ways that make their continued
existence more certain -- a controversial extension of the accepted laws of ecology.
Proponents of Gaia hypothesize that once life takes hold on a planet, it is
very likely that life will remain on the planet in some form over a geological
time scale.

The
habitability of red dwarf planetary systems

Determining the habitability of red
dwarf stars could help determine how common life in the universe is, as
red dwarfs make up between 70 and 90 percent of all the stars in the galaxy. Brown
dwarfs are likely more numerous than red dwarfs. However, they are not
generally classified as stars, and could never support life as we understand
it, since what little heat they emit quickly disappears.

Astronomers for many years ruled out red
dwarfs as potential abodes for life. Their small size (from 0.1 to 0.6 solar
masses) means that their nuclear
reactions proceed exceptionally slowly, and they emit very little light
(from 3% of that produced by the Sun to as little as 0.01%). Any planet in
orbit around a red dwarf would have to huddle very close to its parent star to
attain Earth-like surface temperatures; from 0.3 AU (just inside the orbit of Mercury)
for a star like Lacaille
8760, to as little as 0.032 AU
(such a world would have a year lasting just 6.3 days) for a star like Proxima
Centauri[22] .
At those distances, the star's gravity would cause tidal
lock. The daylight side of the planet would eternally face the star, while
the night-time side would always face away from it. The only way potential
life could avoid either an inferno or a deep freeze would be if the planet had
an atmosphere thick enough to transfer the star's heat from the day side to
the night side. It was long assumed that such a thick atmosphere would prevent
sunlight from reaching the surface in the first place, preventing photosynthesis.

This pessimism has been tempered by
research. Studies by Robert Haberle and Manoj Joshi of NASA's
Ames
Research Center in California have shown that a planet's atmosphere need
only be 15% thicker than Earth's for the star's heat to be effectively carried
to the night side (see Aurelia).
This is well within the levels required for photosynthesis, though water would
still remain frozen on the dark side in some of their models [23] .
Martin Heath of Greenwich
Community College, has shown that seawater, too, could be effectively
circulated without freezing solid if the ocean basins were deep enough to
allow free flow beneath the night side's ice cap. Thus, a planet with deep
enough sea basins and a thick enough atmosphere could, at least potentially,
harbour life in a red dwarf system.

Size is not the only factor in making red
dwarfs potentially unsuitable for life, however. On a red dwarf planet,
photosynthesis on the night side would be impossible, since it would never see
the sun. On the day side, because the sun does not rise or set, areas in the
shadows of mountains would remain so forever, making photosynthesis difficult.
Photosynthesis as we understand it would be further complicated by the fact
that a red dwarf produces most of its radiation in the infrared,
and on the Earth the process depends on visible light.

Red dwarfs are far more variable and
violent than their more stable, larger cousins. Often they are covered in starspots
that can dim their emitted light by up to 40% for months at a time, while at
other times they emit gigantic flares that can double their brightness in a
matter of minutes. Such variation would be very damaging for life, though it
might also stimulate evolution by increasing mutation rates and rapidly
shifting climatic conditions.

There is, however, one major advantage
that red dwarfs have over other stars as abodes for life: they live a long
time. It took 4.5 billion years before humanity appeared on Earth, and life as
we know it will see suitable conditions for as little as half a billion years
more [24] .
Red dwarfs, by contrast, could live for trillions of years, because their
nuclear reactions are far slower than those of larger stars, meaning that life
both would have longer to evolve and longer to survive. Further, while the
odds of finding a planet in the habitable zone around any specific red dwarf
are slim, the total amount of habitable zone around all red dwarfs combined is
equal to the total amount around sun-like stars given their ubiquity [25] .

"Good Jupiters"

"Good Jupiters" are gas giant
planets, like the solar system's Jupiter, that orbit their stars in circular
orbits far enough away from the HZ to not disturb it but close enough to
"protect" terrestrial planets in closer orbit in two critical ways.
First, they help to stabilize the orbits, and thereby the climates, of the
inner planets. Second, they keep the inner solar system relatively free of
comets and asteroids that could cause devastating impacts [26] .
Jupiter orbits the sun at about five times the distance between the Earth and
the Sun. This is the rough distance we should expect to find good Jupiters
elsewhere. Jupiter's "caretaker" role was dramatically illustrated
in 1994 when Comet
Shoemaker-Levy 9 impacted the giant; had Jovian gravity not captured the
comet, it may well have entered the inner solar system.

Early in the Solar System's history,
Jupiter played a somewhat contrary role: it increased the eccentricity of asteroid
belt orbits and enabled many to cross Earth's orbit and supply the planet
with important volatiles. Before Earth reached half its present mass, icy
bodies from the Jupiter–Saturn region and small bodies from the primordial
asteroid belt supplied water to the Earth due to the gravitational scattering
of Jupiter and, to a lesser extent, Saturn[27] .
Thus, while the gas giants are now helpful protectors, they were once
suppliers of critical habitability material.

The galactic neighborhood

Scientists have also considered the
possibility that particular areas of galaxies (galactic habitable zones) are
better suited to life than others; the solar system in which we live, in the Orion
Spur, on the Milky Way galaxy's edge is considered to be in a
life-favorable spot [28] .
Well away from the galactic center, it avoids various dangers:

It is not near the black
hole which is believed to lie at the middle of the galaxy.

The circular orbit of the Sun around
the galactic centre keeps out of the way of the galaxy's spiral arms where
intense radiation and gravitation may lead to disruption.

Relative loneliness is ultimately what a
life-bearing system needs. If Sol were crowded amongst other systems,
neighbours might disrupt the stability of various orbiting bodies (not least Oort
cloud and Kuiper
Belt objects, which can bring catastrophe if knocked into the inner solar
system). Close neighbors also increase the likelihood of being fatally close
to supernova
explosions and pulsars.

Note
2: Life appears to have
emerged on Earth approximately 500 million years after the planet’s
formation. "A" class stars (which live 600 million to 1.2 billion
years) and a small fraction of "B" class stars (which live 10+
million to 600 million) actually fall within this window. At least
theoretically life could emerge in such systems but it would almost
certainly not reach a sophisticated level given these timeframes and the
fact that increases in luminosity would occur quite rapidly. Life around
"O" class stars is exceptionally unlikely, as they live less than
ten million years.

Note
3: That Europa
and to a lesser extent Titan
(respectively, 3.5 and 8 astronomical
units outside our Sun’s putative habitable zone) are considered prime
extraterrestrial possibilities underscores the problematic nature of the HZ
criterion. In secondary and tertiary descriptions of habitability it is
often stated that habitable planets must be within the HZ—this
remains to be proven.

Note
5: Interestingly, there is a "mass-gap" in our solar
system between Earth and the two smallest gas giants, Uranus
and Neptune,
which are both roughly 14 Earth-masses. Assuming this is coincidence and
that there is no geophysical barrier to the formation of intermediary
bodies, we should expect to find planets throughout the galaxy between two
and twelve Earth-masses. If the star system is otherwise favourable, such
planets would be good candidates for life as they would be large enough to
remain internally dynamic and atmosphere retentive over billions of years
but not so large as to accrete the gaseous shell which limits the
possibility of life formation.

Note
6: There is an emerging consensus that single-celled
microorganisms may in fact be common in the universe, especially since
Earth’s extremophiles
flourish in environments that were once considered hostile to life. The
potential occurrence of complex multi-celled life remains much more
controversial. In their work Rare
Earth: Why Complex Life Is Uncommon in the Universe, Peter
Ward and Donald Brownalee argue that microbial life is likely widespread
while complex life is very rare and perhaps even unique to Earth. Current
knowledge of Earth’s history partly buttresses this theory: multi-celled
organisms are believed to have emerged at the time of the Cambrian
explosion close to 600 mya but more than 3 billion years after life
itself appeared. That Earth life remained unicellular for so long
underscores that the decisive step toward complex organisms need not
necessarily occur.

Note
7: According to prevailing theory, the formation of the Moon
commenced when a Mars-sized body struck the Earth a glancing collision late
in its formation, and the ejected material coalesced and fell into orbit
(see giant
impact hypothesis). In Rare Earth Ward and Brownalee emphasize
that such impacts ought to be rare, reducing the probability of other
Earth-Moon type systems and hence the probability of other habitable
planets. Other moon formation processes are possible, however, and the
proposition that a planet may be habitable in the absence of a moon has not
been disproven.